Method for producing ring-shaped fibrous structures, in particular for making parts in composite material

ABSTRACT

To make an annular fiber structure, a strip-shaped fabric ( 50 ), is used which is made up of two superposed unidirectional sheets, with the directions of the sheets forming opposite angles relative to the longitudinal direction of the strip, the two sheets being bonded together so as to form deformable elementary meshes, the fabric being wound while being deformed so as to transform it into a flat helix, the elementary meshes deforming in such a manner that variation in mass per unit area between the inside and outside diameters of the turns remains small, and the flat turns are pressed against one another.

CROSS REFERENCE TO RELATED APPLICATIONS

This is a National Entry from PCT Application PCT/FR98/00598, filed Mar.25, 1998, which claims priority from French Application No. 97/03833,filed Mar. 28, 1997.

BACKGROUND OF THE INVENTION

The present invention relates to a method of making annular fiberstructures, in particular preforms for manufacturing annular parts ofcomposite material.

A particular but non-exclusive field of application of the inventionlies in making annular preforms for the manufacture of brake disks orclutch disks out of composite material, and in particular carbon-carbon(C/C) composite material.

Annular parts of composite material, such as brake disks or clutchdisks, are constituted by a fiber reinforcing structure or “preform”which is densified by a matrix. For C/C composite disks, the preform ismade of carbon fibers or of fibers made of a carbon precursor which istransformed into carbon by heat treatment after the preform has beenmade. A particular carbon preform that is available in fiber form ispre-oxidized polyacrylonitrile (PAN). The preform can be densifiedeither by a liquid-impregnation process using a liquid precursor forcarbon, e.g. a resin, and then transforming the precursor by heattreatment, or else by chemical vapor infiltration, or indeed bycalefaction. For calefaction, the preform is immersed in amatrix-precursor liquid and the preform is heated, e.g. by contact withan induction core or by direct coupling with an induction coil, so thatthe precursor is vaporized on making contact with the preform and caninfiltrate to form the matrix by depositing within the pores of thepreform.

A well known method of making fiber preforms for parts made of compositematerial consists in superposing and needling together layers or pliesof a two-dimensional fiber fabric. By way of example, the fiber fabriccan be a woven cloth. The cloth may optionally be covered in a web offibers for producing the fibers that are suitable for being displaced byneedles through the superposed plies; this applies in particular whenthe cloth is made of fibers that are difficult to needle without beingbroken, and in particular carbon fibers. Such a method is described inparticular in documents FR-A-2 584 106 and FR-A-2 584 109 respectivelyfor making preforms that are plane and for making preforms that arebodies of revolution.

An annular preform for a disk can be cut out from a thick plate made upof layers that have been superposed flat and needled together. The lossof material then amounts to nearly 50% which, for preforms made ofcarbon fibers or of carbon precursor fibers, constitutes a very largeexpense.

In order to reduce this loss, proposals are made in document EP-A-0 232059 to build up a preform by superposing and needling together annularlayers, each of which is formed by assembling together a plurality ofsectors. The sectors are cut out from a two-dimensional fabric. The lossof material is less than when cutting out entire rings, but it is stillnot negligible. In addition, the method is rather difficult to implementand to automate, in particular because of the need to position thesectors correctly while ensuring that they are offset from one layer toanother so as to avoid superposing lines of separation between sectors.

It might be envisaged that annular preforms could be cut out fromsleeves made by rolling a strip of cloth onto a mandrel whilesimultaneously needling it, as described in above-mentioned documentFR-A-2 584 107. That method is relatively easy to implement withoutsignificant loss of fiber material. However, in an application tofriction disks, and contrary to the other embodiments described above,the plies of the preform are then disposed perpendicularly to thefriction faces, and in some cases that configuration is not optimal.

Another known technique for making fiber preforms for annular parts madeof composite material consists in using a textile product in the form ofa helical or spiral strip, which product is wound as flat superposedturns. The textile product can be a woven cloth made up of helical warpthreads and of radial weft threads.

As described in documents FR-A-2 490 687 and FR-A-2 643 656, the spiralhelical shape is given to the cloth by making use of a frustoconicalroller for the warp threads being reeled out from individual spoolsmounted on a creel. In a cloth made in that way, the spacing between theradial weft threads increases across the width of the helical clothbetween the inside diameter and the outside diameter.

In order to conserve a substantially uniform nature for the cloth acrossits entire width, it is proposed in the two above-mentioned documents tointroduce additional weft threads that extend over a portion only of thewidth of the cloth, starting from its outside diameter. That solutiongives rise to significant extra cost in manufacturing the cloth, and isa non-negligible source of defects. Another solution, described inpatent application FR 95 14 000, consists in increasing the mass perunit area of the warp of the helical cloth between the inside diameterand the outside diameter thereof so as to ensure that in terms ofdensity per unit volume of the preform, the decrease in weft density iscompensated approximately. Although less expensive than increasing thedensity of weft fibers towards the outside diameter, that solutionnevertheless remains rather complex since it requires the use of warpthreads of varying weight and/or varying mass per unit area between theinside diameter and the outside diameter of the cloth.

In yet another known technique, fiber preforms for annular parts made ofcomposite material, and in particular for brake disks, are made bywinding flattened tubular braids helically. The tubular braids can berectilinear, as described in document EP-A-0 528 336. The braids arethen deformed so as to be wound into a helix. Longitudinal threads canbe added during manufacture of the braid so as to improve thedimensional stability of the preform and so as to compensate forvariation in density per unit area between the inside diameter and theoutside diameter of the wound flattened braid. Proposals have also beenmade in document EP-A- 0 683 261 to use helical tubular braids. Thatmakes it possible to overcome the limits on deformability of rectilineartubular braids when they are being wound into a helix. Nevertheless, thevariation in density per unit area still needs to be compensated byadding longitudinal fibers or by juxtaposing a plurality of flattenedbraids of small width between the inside diameter and the outsidediameter. Those solutions make preform manufacture relatively complex,and thus expensive, without providing a completely satisfactory solutionto the problem of variation in density per unit area.

BRIEF SUMMARY OF THE INVENTION

Thus, an object of the present invention is to provide a method thatenables annular preforms to be made for composite material parts withoutgiving rise to significant wastage of material and while conservingsubstantially constant density per unit area between the inside diameterand the outside diameter of the structure.

Another object of the present invention is to provide such a method inwhich the cost of implementation is significantly less than that of theprior art methods that enable similar results to be obtained.

To this end, the invention provides a method of making an annular fiberstructure by winding a flat helix of a fiber fabric in the form of adeformable strip, the method steps comprising in:

supplying a fiber fabric in the form of a deformable strip made up oftwo superposed unidirectional sheets, each constituted by mutuallyparallel fiber elements, the direction of the fiber elements is onesheet forming angles of opposite signs with respect to the direction ofthe fiber elements in the other sheet and relative to the longitudinaldirection of the strip, and the two sheets being bonded together so asto form deformable elementary meshes;

deforming the strip shaped fabric into a helix having turns by modifyingthe shape of the elementary meshes so that their radial size increasestowards the inside diameter of the helix, whereby the variation in massper unit area between the inside and outside diameters of the turns isminimized; and

winding the deformed fabric into a flat helix applying the deformedturns flat against one another by so as to obtain an annular fiberstructure whose radial dimension between its inner diameter and outerdiameter corresponds to the width of the deformed strip-shaped fabric.

Advantageously, the directions of the fiber/elements in the sheetsrelative to the longitudinal direction of the strip form angles havingabsolute values that preferably lie in the range of 30° to 60°, so as tomaintain the ability of the elementary meshes to deform in thelongitudinal direction and in the transverse direction. In a preferredembodiment, these angles are equal to +45° and −45°. The sheets arebonded together while preserving the ability of the elementary meshes todeform at their apexes, e.g. by sewing or by knitting, using threadsthat pass from one face of the fabric to the other, or indeed bypreneedling or by localized needling.

Such a fabric is particularly advantageous because of its ability todeform which enables it to be wound as a flat helix without formingthickenings or wrinkles on its surface and with substantially uniformdistribution of the fiber elements in the sheets, thereby giving thehelix a density per unit area whose variation between the inside andoutside diameters can remain within limits that are acceptable, withoutthere being any need for compensation.

Also advantageously, the flat superposed turns formed by winding thefabric into a helix are bonded to one another. Bonding between the turnscan be performed, for example, by needling. The needling can beperformed after winding and optional compression of the annularstructure, or else while winding is taking place.

The strip-shaped fabric can be deformed by passing between two rotarydisks with the longitudinal edges of the fabric being held between thedisks, e.g. by clamping, or else the fabric can be deformed by passingover at least one frustoconical roller.

It is thus possible to form an annular fiber structure without loss offiber material and while conserving fiber density that varies littlebetween the inside diameter and the outside diameter without any need tointroduce additional elements as in prior art methods, thereby greatlysimplifying implementation.

BRIEF DESCRIPTION OF THE FIGURES

The invention will be better understood on reading the followingdescription given by way of non-limiting indication with reference tothe accompanying drawings, in which:

FIG. 1 is a highly diagrammatic view of an installation enabling a fibertexture to be made in the form of a deformable strip suitable for use inimplementing the method of the invention;

FIGS. 2, 3A, 3B, and 3C are views illustrating one way in which a fiberfabric suitable for use in implementing a method of the invention can bebonded by knitting;

FIGS. 4, 5A, 5B, and 5C are views showing another way in which a fiberfabric suitable for implementing a method of the invention can be bondedby knitting;

FIG. 6 is a diagrammatic detail view showing how a fiber fabric, such asthat made by the installation of FIG. 1, deforms when it is wound flatin a helix;

FIGS. 7A and 7B are diagrammatic views showing a device for helicallywinding a fiber fabric to implement a method of the invention;

FIGS. 8A and 8B are diagrammatic views showing two other devices forhelically winding a fiber fabric to implement a method of the invention;

FIG. 9 is a diagrammatic view showing an implementation of a method ofmaking an annular fiber structure in accordance with the invention; and

FIG. 10 is a diagrammatic view showing another implementation of amethod of making an annular fiber structure in accordance with theinvention.

DETAILED DESCRIPTION OF THE INVENTION

The fiber fabric used in a method of the invention is made bysuperposing and binding together two unidirectional sheets made up ofmutually parallel fiber elements.

In well known manner, a unidirectional sheet can be obtained byspreading out and laying a tow or cable, or, as described in thedetailed description below, by paralleling threads taken from differentspools.

It will be observed that a method of making a multi-axial fiber fabricfrom unidirectional sheets obtained by spreading out tows is describedin the French patent application filed on Mar. 28, 1997 under the No.97/03832 and entitled “A method and a machine for making multi-axialfiber sheets”, the contents of which is incorporated herein byreference.

FIG. 1 shows very diagrammatically an installation that receives twounidirectional sheets 10, 12 made up of threads, and that produces afabric in the form of a strip by superposing two sheets that make anglesof opposite signs with the longitudinal direction of the strip, and inthe example these angles are equal to +45° and −45°.

The fibers constituting the unidirectional sheets 10 and 12 are of amaterial that is selected as a function of the use intended for thefabric in strip form. The fibers can be organic or inorganic, e.g.carbon fibers or ceramic fibers, or fibers made of a precursor forcarbon fibers or ceramic fibers. It will be observed that the fibersconstituting the two sheets can be of different kinds. It is alsopossible to use fibers of different kinds in each of the sheets.

The strip is formed by bringing in successive segments of the sheet 10that is at an angle of +45° relative to the longitudinal direction ofthe strip that is to be made, and by juxtaposing these segments in saiddirection. Each segment is brought in over a length such that it extendsfrom one longitudinal edge of the strip to its other longitudinal edge.In similar manner, successive segments of the sheet 12 are brought in atan angle of −45° relative to the longitudinal direction of the strip tobe made and they are juxtaposed, with the sheet segments 12 being placedover the sheet segments 10.

In the example shown, the threads 11, 13 constituting each of the sheets10, 12 are tensioned between two spiked endless chains 20, 22 that aredriven synchronously. The ends of the sheets 10 and 12 are guided byrespective carriages 14 and 16 that receive the threads 11 and 13 fromrespective spools (not shown) and that are driven back and forth betweenthe longitudinal edges of the strip to be made. At each end of thestroke of the carriages, the sheets are turned around the spikes of thecorresponding spiked chain. The spiked chains 20 and 22 are caused toadvance continuously or discontinuously in time with the sheets 10 and12 being brought in so as to cause successive sheet segments to bejuxtaposed. An installation of this type is known, e.g. from documentU.S. Pat. No. 4,677,831, so a more detailed description is notnecessary.

The strip formed by superposing the sheets 10 and 12 is taken off thespiked chains 20 and 22 at the downstream end of their top edges foradmission into a bonding device 30. In the example shown, bonding isperformed by needling by means of a needle board 32 which extends overthe entire width of the strip being formed, the strip passing over aperforated plate 34 whose perforations are situated in register with theneedles of the board 32. The distribution of the needles on the needleboard 32 is determined so as to perform needling that is localized sothat the bonding between the two sheets defines individual stitches thatare deformable, e.g. in parallelogram manner.

The bonding between the sheets of the resulting strip-shaped fiberfabric 50 confers sufficient cohesion to enable the fabric to be storedon a roll 38 driven by a motor 48 synchronously with the spiked chains20 and 22. Between the bonding device 30 and the roll 38, the edges ofthe strips 50 are cut by means of rotary dies 36 a and 36 b.

FIGS. 2, 3A, and 3B show a preferred variant implementation of bondingbetween the sheets. In this variant, bonding is performed not byneedling, but by knitting. The superposed sheets taken from the spikedchains 20 and 22 are received by a knitting machine 42 which performsknitting, i.e. it makes a two-dimensional structure, by means of athread that passes from one face to the other of the fabric 50 (FIG. 2).FIG. 3A shows in detail the knitting stitch 44 used, while FIGS. 3B and3C show the front and the back faces of the fabric 50 bonded by theknitting.

As shown in FIG. 3A, the knitting stitch forms interlaced loops 44 athat are elongate in the longitudinal direction of the fabric 50,forming a plurality of parallel rows, together with V-shaped orzigzag-shaped paths 44 b interconnecting the loops in adjacent rows. Thefabric 50 is situated between the paths 44 b situated on the front face(FIG. 3B) and the loops 44 a situated on the back face (FIG. 3C), givingthe knit the appearance of a zigzag stitch on one face and a chainstitch on the other face. The knitting stitch covers a plurality ofthreads in each sheet, the number depending on the chosen gauge.

The bonding points between the zigzag paths 44B and the loops 44 a, suchas the points A, B, C, and D in FIGS. 3B and 3C defined the apexes ofindividual deformable meshes. In this case, both the meshes defined bythe knitting stitch are deformable, as are the meshes defined by thecrossover points between the threads of the sheets that form deformableparallelograms.

FIG. 4 shows another variant in which the bonding between the sheets isachieved by knitting. The superposed sheets taken from the spiked chains20 and 22 are received by a knitting machine 46 which bonds together thesheets in a plurality of lines parallel to the longitudinal edges of thefabric 50.

As shown in FIG. 5A, each knitting stitch 48 is a chain stitch withloops 48 a linked via rectilinear segments 48 b, the fabric 50 issituated between the segments 48 b that are visible on the back face ofthe fabric (FIG. 5B) and the loops 48 a that are visible on its frontface (FIG. 5C).

The knitting stitch for the embodiments of FIGS. 2 and 4 can be made ofa sacrificial material, i.e. a material that can subsequently beeliminated without damaging the fibers constituting the sheets. Forexample it is possible to use threads of a material suitable for beingeliminated by heat without leaving any residue, or threads of a materialthat is suitable for being eliminated by a solvent, for examplewater-soluble polyvinyl alcohol threads.

It is also possible to use a knitting thread made of a material that iscompatible with the intended subsequent use of the fabric. When thefabric is intended for making preforms for use in the manufacture ofcomposite material parts, the knitting or sewing thread may be made of amaterial compatible with the matrix material of the composite material,i.e. preferably of the same kind as or miscible in the matrix withoutreacting chemically therewith.

Other methods of bonding by knitting or indeed by sewing could also beselected.

The resulting strip-shaped fabric is particularly advantageous becauseof its ability to deform which enables it to be wound flat and helicallywithout giving rise to surface deformation (wrinkles or slippage), withthis being because the elementary meshes 52 of the texture 50 behavelike deformable parallelograms whose deformation is not constrained bythe selected method of bonding, the method of bonding by knitting asshown in FIGS. 2, 3A, 3B, and 3C being the method that is preferred inthis respect.

During winding (FIG. 6), the elementary meshes 52′ situated close to theinside diameter of the helix being formed deform by being elongatedradially and by shrinking longitudinally, while the elementary meshes52″ situated in the vicinity of the outside diameter of the helix deformby shrinking in the radial direction and lengthening in the longitudinaldirection. As a result, the density of fibers per unit area remainssubstantially constant or varies only little between the inside diameterand the outside diameter, which is particularly advantageous for makinghomogenous preforms for use in the manufacture of composite materialparts. In FIG. 6, chain-dotted line 54 shows the deformation of one ofthe initial directions of the strip 50.

When bonding is by knitting or by sewing, the deformation of theelementary meshes formed by the threads of the fabric is accompanied bydeformation of the knitting or sewing stitches. Thus, for the knittingstitch of FIGS. 3A to 3C, the deformation gives rise to lengthening orshortening of the portions of the thread forming the chain-stitch loopsand by opening or closing of the angles formed by the zigzag paths.

The fabric 50 can be rolled into a flat helix with deformation bycausing the fabric to pass between two annular disks or plates 60 and 62while holding the fabric along its longitudinal edges between the disks(FIG. 7A). The fabric can be held, for example, by clamping its edgesbetween circular ribs 64 and 66 formed on the inside faces of the disks60 and 62, or at least on the inside face of one of the disks (FIG. 7B).

In another embodiment, the fabric is wound and deformed by causing it topass over at least one frustoconical roller. The number of rollers usedand their angles at the apex are selected as a function of the desireddegree of deformation. In the example shown in FIG. 8A, two identicalfrustoconical rollers 70 and 72 are used which are rotated by respectivemotors (not shown). The fabric is caused to fit closely over a fractionof the periphery of at least one of the rollers.

In the example of FIG. 8B, the fabric is caused to pass between a firstrotary frustoconical roller 74 and a smooth presser plate 75, and alsobetween a second rotary frustoconical roller 76 and a smooth presserplate 77. The rollers are rotated by respective motors (not shown) andthey deform the fabric by friction.

It is possible to use a single frustoconical roller against which thefabric is pressed. Under such circumstances, it is the smaller circledescribed by one of the edges of the fabric on the frustoconical rollerthat defines the inside diameter of the helix.

An annular fiber structure can be built up by superposing the flat turnsformed by winding the fabric 50 helically and by bonding the turns toone another by needling as winding takes place (FIG. 9). This can beperformed continuously while deforming the fiber fabric into a flathelix or after intermediate storage thereof.

The fabric 50 as deformed, e.g. by passing between two disks as shown inFIG. 5, is wound into superposed flat turns on a turntable 80. Theturntable 80 is mounted on a vertical shaft 82 secured to a support 84.The support 84 also carries a motor 86 which drives the turntable 80 soas to rotate it about its vertical axis 90 (arrow f1) via a belt 88.

The assembly comprising the support 84 and the turntable 80 isvertically movable along a fixed central guide tube 92 having the sameaxis 90. At its top end, the tube 92 supports the device for deformingthe strip into a helix. The support 84 stands on vertical telescopicrods 94, with vertical displacement of the support 84 being under thecontrol of one or more actuators 96.

As the strip 50 is wound flat onto the rotating turntable 80, it isneedled by means of a board 100 carrying needles 102 and driven withvertical reciprocating motion. The motion of the needle board is drivenby a motor 104 via a crank and connecting rod type transmission. Themotor 104 is carried by the support 84.

The strip 50 is needled at a density per unit area and at a depth thatare substantially constant. To obtain a constant density for the strokesof the needles 102 over the entire area of an annular turn of the strip50, the needle board 100 is sector-shaped, corresponding to a sector ofan annular layer of the cloth, with the needles being distributeduniformly over said sector, while the turntable 80 supporting thestructure 110 that is being built up is itself driven to rotate at aspeed that is constant.

The depth of needling, i.e. the distance the needles 102 penetrate oneach stroke into the structure 110 is maintained substantially constantand is equal, for example, to the thickness formed by a plurality ofsuperposed layers of cloth. To this end, as the strip 50 is being woundon the turntable 80, the turntable is displaced vertically downwardsthrough the appropriate distance to ensure that the relative positionbetween the surface of the preform and the needle board at one end ofits vertical stroke remains unaltered. Once the preform 110 has beenbuilt up, after the last turn of the strip 50 has been put into place, aplurality of needling passes are performed while continuing to cause theturntable 80 to rotate so that the density of needling per unit volumein the surface layers of the cloth remains substantially the same aswithin the remainder of the preform. During at least a portion of thesefinal needling passes, the turntable can be caused to move downwardsprogressively, as during the preceding stages. This principle ofneedling to constant depth by progressively lowering the preform supportand by applying final needling passes is known, and in particular it isdescribed in above-mentioned document FR-A-2 584 106. In addition, theturntable 80 is coated in a protective layer 106 into which the needlescan penetrate without being damaged while they are needling the initialturns of the strip 50. The protective layer 106 can be constituted by abase felt, e.g. a polypropylene felt, covered in a sheet of plasticsmaterial, e.g. of polyvinyl chloride, thereby preventing the needlesduring their upstroke from entraining fibers from the base felt into thepreform 110.

In another embodiment of the fiber structure 110, the turns formed byhelically winding the deformed fabric are applied against one another,and the fiber structure is compressed by means of tooling comprising abase plate 130 and a top plate 132 (FIG. 10). Compression is performedso as to obtain the desired density of fibers per unit volume. The turnscan then be bonded together by needling using a needle board 134 whoseneedles 136 pass through perforations in the top plate 132 and penetrateall the way through the thickness of the structure 110. Perforations canalso be formed in the bottom plate 130 in register with the needles.

An annular fiber structure obtained as described above is suitable foruse as a preform in manufacturing an annular part out of compositematerial, e.g. a brake disk.

When the unidirectional sheets have been bonded by means of a thread ofsacrificial material, the thread is eliminated by dissolving or by heattreatment prior to the preform being densified.

When the material constituting the fibers of the resulting fiberstructure is a precursor for the fiber reinforcement of the compositematerial, the precursor is transformed prior to the preform beingdensified, or while its temperature is being raised prior todensification.

The preform is densified in conventional manner by a liquid process orby chemical vapor infiltration so as to form a deposit of materialconstituting the desired material within the accessible pores of thepreform.

Although the above description relates to using a deformable fiberfabric made up of two bonded-together unidirectional sheets formingangles of +45° and −45° relative to the longitudinal direction of thesheet, it will be understood that the method of the invention can beimplemented with deformable strips in which the two unidirectionalsheets form angles of opposite signs having absolute values that candiffer from 45°, and that can possibly differ from each other.Nevertheless, in order to conserve sufficient deformation capacity forthe mesh, it is preferable for said angles to have an absolute valuelying in the range 30° to 60°, and also preferably said angles shouldhave the same absolute value so as to conserve symmetry in thedeformable strip.

In addition, it is assumed above that the fiber strip is wound into ahelix on leaving the laying installation of FIG. 1. In a variant, andeven preferably, when the radial size of the annular preforms to be madeis not too large, the fiber strip leaving the laying installation isinitially subdivided into a plurality of deformable strips, notnecessarily of the same width, by being cut parallel to the longitudinaldirection.

What is claimed is:
 1. A method of making an annular fiber structure bywinding a flat helix of a fiber fabric in the form of a deformablestrip, the method steps comprising: supplying a fiber fabric in the formof a deformable strip made up of two superposed unidirectional sheets,each constituted by mutually parallel fiber elements, the direction ofthe fiber elements in one sheet forming angles of opposite signs withrespect to the direction of the fiber elements in the other sheet andrelative to the longitudinal direction of the strip, and the two sheetsbeing bonded together so as to form deformable elementary meshes;deforming the strip-shaped fabric into an helix having turns bymodifying the shape of the elementary meshes so that their radial sizeincreases towards the inside diameter of the helix turns, whereby thevariation in mass per unit area between the inside and outside diametersof the turns is minimized; and winding the deformed fabric into a flathelix by applying the deformed turns flat against one another so as toobtain an annular fiber structure whose radial dimension between itsinner diameter and outer diameter corresponds to the width of thedeformed strip-shaped fabric.
 2. A method according to claim 1, whereinsaid angles of opposite signs have same absolute value.
 3. A methodaccording to claim 1, wherein said superposed unidirectional sheets arebonded together by needling.
 4. A method according to claim 1, whereindeforming the strip-shaped fabric into an helix includes passing thefabric over at least one frustoconical roller.
 5. A method according toclaim 1, wherein the strip-shaped fabric is subdivided into a pluralityof deformable strips by longitudinally cutting up a strip of greaterwidth prior to deforming each strip and winding each deformed strip intoa flat helix to obtain a respective annular fiber structure.
 6. A methodaccording to claim 1, wherein said angles of opposite signs have anabsolute value lying in the range of 30° to 60°.
 7. A method accordingto claim 6, wherein said absolute value is equal to 45°.
 8. A methodaccording to claim 1, wherein said superposed unidirectional sheets arebonded together by knitting.
 9. A method according to claim 8, whereinsaid knitting uses a knitting stitch that forms a zigzag on one face ofthe strip-shaped fabric and a chain stitch on the face opposite to saidone face.
 10. A method according to claim 8, wherein said sheets arebonded together by a thread of sacrificial material.
 11. A methodaccording to claim 1, wherein said superposed unidirectional sheets arebonded together by sewing.
 12. A method according to claim 11, whereinsaid sheets are bonded together by a thread of sacrificial material. 13.A method according to claim 1, wherein said superposed flat turns arebonded to one another.
 14. A method according to claim 13, wherein saidsuperposed flat turns are bonded to one another by needling.
 15. Amethod according to claim 14, wherein said needling is performedprogressively as the fabric is wound into a flat helix.
 16. A methodaccording to claim 1, wherein deforming the strip-shaped fabric into anhelix is performed by passing the strip-shaped fabric between two rotarydisks between which the fabric is held along its longitudinal edges. 17.A method according to claim 16, wherein the strip-shaped fabric isclamped along its longitudinal edges between the disks.